Magnetic field sensors, such as magneto-resistive (XMR) sensors, are used in a variety of applications for sensing the rotation of a wheel and/or shaft, such as in anti-lock braking systems, crank-shaft sensors, cam-shaft sensors, etc., and for sensing vertical and/or angular movement, for example. XMR sensors include, for example, anisotropic magneto-resistive (AMR) type sensors, tunneling magneto-resistive (TMR) sensors, giant magneto-resistive (GMR) sensors, and colossal magneto-resistive (CMR) sensors. Typically, XMR type magnetic field sensors include one or more sensor elements, such as GMR sensor elements, which are formed as part of a semiconductor die which further includes integrated circuitry for evaluating parameters of the sensor (e.g. resistance of the GMR sensor elements). In turn, the semiconductor die is typically bonded to a carrier, such as a copper leadframe, for example, to form a magnetic field sensor package.
In the case of speed and/or angle sensors, the package further includes a permanent magnet which provides a back bias magnetic field for the XMR sensor elements. The magnetic field sensor is placed in front of a magnetically permeable toothwheel, the rotation of which generates a sinusoidally varying magnetic field on the magnetic field sensor. The XMR sensor elements detect variations in the component of the magnetic field parallel to the surface of the magnetic field sensor, with the detected magnetic field variations providing information about the angular position, rotational direction, and rotational speed of the toothwheel.
Often, the permanent magnetic is a permanent magnet attached to the backside of the magnetic field sensor, such as to a surface of the leadframe opposite the semiconductor die. However, attaching a permanent magnet in this fashion has a number of disadvantages. For example, the permanent magnet has position tolerances with respect to the magnetic field sensor package since the permanent magnet is typically attached after the completion of the fabrication of the magnetic field sensor package. Also, the glue used to attach the permanent magnet to the magnetic field sensor has to be selected carefully since the sensor assembly is typically exposed to a wide temperature range (e.g., −50° C.-170° C.). Additionally, the permanent magnet is typically attached to the magnetic field sensor package by someone other than the semiconductor manufacturer who fabricated the magnetic field sensor package, so that the permanent magnet is attached to the magnetic field sensor package after the final test of the magnetic field sensor package at the semiconductor manufacturer. Furthermore, the semiconductor manufacturer typically tests the magnetic field sensor package at several temperatures. However, the permanent magnet is typically not tested at several temperatures since the thermal mass of the entire sensor assembly is usually too large to perform an economical multi-temperature test.
In attempts to overcome such disadvantages, in lieu of attaching a permanent magnet to the backside of the leadframe, the semiconductor die and leadframe of some XMR sensors are embedded within a molded magnetic material. While such an approach eliminates many of the above described problems, it has been found that the ferromagnetic material of the leadframe (e.g. copper) produces distortions in the magnetic field or flux lines at the XMR elements which results in a horizontal component of the magnetic field, parallel to the surface of the magnetic field sensor (i.e. the sensed component), having a magnitude that result the XMR sensors being in saturation, thereby rendering the sensor inoperable.
For these and other reasons, there is a need for the present invention.